When shopping around for what cheese/toppings combo I wanted to use for my own sandwich at our grilled cheese party, I happened across pesto gouda combo cheese at Trader Joe's. I figured the bright green color would make for a fun looking grilled cheese (and by fun I mean that it would resemble green eggs & ham). I also picked up some raspberries to toss in to try making it more of a sweet grilled cheese. People questioned these choices, but I love how this sandwich turned out- especially the tart flavor from the berries. If you can't find pesto gouda you could always use regular gouda and add in a couple tablespoons of regular pesto.
Check out some other interesting grilled cheese flavor combos at the bottom of this post!